1. Field of the Invention
The invention concerns a process for the electrostatic printing of functional materials configured as liquid toners on relatively thick glass plates for various manufacturing applications.
2. Description of the Related Art
Flat panel displays or wall type television sets have been discussed in the prior art literature for about forty years, but few have been produced. As of mid 1998 there were three primary flat panel technologies for flat panel displays:
a. Field Emission Displays (FED""s.)
b. Plasma Displays
c. Active Matrix Liquid Crystal Displays (AMLCD)
Field emission displays are a relatively new technology. They consist of an array of field emission points in a vacuum, spraying electrons onto a phosphor screen. With three color dots on the screen and addressibility of the emitting points, one has a full color display.
The Plasma displays have been produced for about 25 years, mostly as a single color orange neon xe2x80x9cglow dischargexe2x80x9d. In the last 10 years, UV light from this discharge has been xe2x80x9charnessedxe2x80x9d to excite three color phosphors to produce a color plasma displays. 40xe2x80x3 diagonal displays have been recently announced, but their cost is about $10,000.
Active matrix liquid crystal displays have been intensively developed for production. Billions of dollars have been spent on their development over the last 20 years, but the results have been only an expensive small display (10.4 inch diagonal) for lap top computers. The 1996 cost of a 10.4xe2x80x3 display is about $500. Wall type TV units, 20xe2x80x3 diagonal or so, are perhaps available after the year 2000, but very expensive.
The reason for the small size/high cost of production are the currently used manufacturing techniques. These include:
a. photolithography or the patterning of photo sensitive resists and the xe2x80x9cwashingxe2x80x9d and etching processes that are attendant to them.
b. the silk screen printing of relatively large area features (30xcexc or more)
c. the low pressure sputtering processes for coating glasses with metals like aluminum or indium/tin oxide (ITO), a transparent electrode or dielectrics like SiO2.
In all cases the process has many steps, many in which the glass has to be heated and then cooled back to room temperature before the next step. Each of these steps requires a large piece of capital equipment in a class 100 clean room whose capital cost is $500 per square foot for the room itself. The capital equipment runs the gamut from a $40,000 liquid etcher, or developer, to a $2.5M stepper to a $4M sputtering cluster (six to eight vacuum chambers that accept 1 mxc3x971 m glass).
There is xe2x80x9csuitexe2x80x9d of expensive capital equipment in a typical $500 per square foot clean room so that the cost of a modern AMLCD production facility is approximately $500 Million. None of the raw materials for the displays, including the glass, glass powder or frit, phosphor, aluminum or nickel, resin or color filter resins are very expensive. Costs are incurred by the capital equipment and low yield of a complex process with many steps.
What is needed is a simpler manufacturing process with fewer steps that requires less capital equipment, does not involve heating and cooling within the imaging step as this dimensionally distorts the glass substrate by thermal expansion, and is implementable with relatively inexpensive machinery, i.e. no vacuum chambers, laser exposure steps etc.
Electrostatic printing has been used for color proofing in Du Ponts EMP process during the late 1980""s. Du Pont used the electrostatic printing which is described by Reisenfield in U.S. Pat. No. 4,732,831. It used liquid toners that were transferred directly to a smooth, coated sheet of paper.
The transfer of liquid toner, which is important to this invention, was disclosed by Bujese in U.S. Pat. No. 4,879,184 and U.S. Pat. No. 4,786,576. These documents teach the transfer of liquid toners across a finite mechanical gap, typically 50xcexc to 150xcexc. This technology has been applied where toner, with etch resist properties, was transferred to copper clad glass epoxy boards.
Other prior work related to the printing plate and xe2x80x9cgap transferxe2x80x9d includes M. B. Culhane (Defensive Publication# T869004, Dec. 16, 1969) and Ingersol and Beckmore to the electrostatic printing plate (U.S. Pat. No. 3,286,025 and RE 29,357; RE 29,537 respectively).
Briefly described, the present invention teaches a technique for the electrostatic printing of functional materials on glass to produce various xe2x80x9cmicrostructuresxe2x80x9d like ribs or to electrodes, spacers, filters etc. by a copy machine type of device at rates from 0.1 to 1.0 m/sec. In some cases there is a later step of sintering or electroless plating, but this is xe2x80x9cafter the factxe2x80x9d in that dimensional accuracy was previously determined by the printing step done at room temperature. The functional materials include metals, dielectrics, phosphors, catalytic seed materials, etc. configured as liquid toners. Since the substrate material is glass it presents special requirements:
1. It is mechanically of irregular shape (i.e. it is wedge shaped in orthogonal directions and its thickness has considerable variation); and,
2. It is a very thick material to be electrostatically imaged compared to the paper or polymeric films printed on by copiers or laser printers.
For this reason the invention uses liquid toners (dispersions of solid particles; metal, glass, etc.) that can be electrostatically transferred across a significant mechanical fluid filled gap.
While the xe2x80x9cgap transferxe2x80x9d technique just described is useful in production machinery handling 1.0 m by 1.4 m plates, there are many situations where the printing capability on a relieved surfaced is a significant advantage, and the magnitude of surface relief can be quite substantial, of the order of 0.1 mm or 100xcexc or more.
The electrostatic printing function is typically done in one process step. Afterwards the particulate mass is fused into a solid structure with a subsequent heating step. In one embodiment of the invention, catalytic seed toners are printed followed by xe2x80x9celectrolessxe2x80x9d plating steps where metals like copper, or nickel, are deposited on the glass.
Finally, there are certain partially manufactured products like color filters or CRT face plates which can be used in a process wherein the final part plays the role of a printing plate to print on itself. This is very simple and therefore inexpensive process which contains a xe2x80x9cself-healingxe2x80x9d feature. Imperfections in the semi finished parts are automatically overprinted with the liquid toner.
The invention may be more fully understood by referring to the following drawings.